A solar cell converts solar energy directly to DC electric energy. Generally configured as a photodiode, it permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier (electrons or holes (a lack of electrons)) may be extracted as current. And like most other diodes, photodiodes are formed by combining p-type and n-type semiconductors to form a junction.
Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
Referring now to FIG. 1, a simplified diagram of a conventional solar cell is shown. In general, a moderately doped diffused emitter region 108 is generally formed above a relatively light and counter-doped diffused region absorber region 110. In addition, prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, the set of metal contacts, comprising front-metal contact 102 and back surface field (BSF)/back metal contact 116, are formed on and fired into silicon substrate 110.
In a common configuration, a light n-type phosphorous-doped diffused region 108 (generally called the emitter or field), is formed by exposing the boron-doped substrate to POCl3 (phosphorus oxychloride) ambient to form phosphosilicate glass (PSG) on the surface of the wafer. The reduction of phosphorus pentoxide by silicon releases phosphorus into the bulk of the substrate and dopes it. The reaction is typically:4POCl3(g)+3O2(g)→2P2O5(l)+6Cl2(g)  [Equation 1A]2P2O5(l)+5Si(s)→5SiO2(s)+4P(s)  [Equation 1B]Si+O2→SiO2  [Equation 2]
The POCl3 ambient typically includes nitrogen gas (N2 gas) which is flowed through a bubbler filled with liquid POCl3, and a reactive oxygen gas (reactive O2 gas) configured to react with the vaporized POCl3 to form the deposition (processing) gas. In general, the reduction of P2O5 to free phosphorous is directly proportional to the availability of Si atoms.
During the diffusion process, the substrates are loaded in either a back-to-back configurations with two substrates per slot, or in a single wafer per slot configuration, such that all substrate surfaces exposed to the furnace ambient are doped with phosphorus.
Prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, residual surface glass (PSG) formed on the substrate surface during the POCl3 deposition process may be removed by exposing the doped silicon substrate to an etchant, such as hydrofluoric acid (HF). The set of metal contacts, comprising front-metal contact 102 and BSF (back surface field)/back metal contact 116, are then sequentially formed on and subsequently fired into doped silicon substrate 110.
The front metal contact 102 is commonly formed by depositing an Ag (silver) paste, comprising Ag powder (about 70 to about 80 wt % (weight percent)), lead borosilicate glass (frit) PbO—B2O3—SiO2 (about 1 to about 10 wt %), and organic components (about 15 to about 30 wt %). After deposition the paste is dried at a low temperature to remove organic solvents and fired at high temperatures to form the conductive metal layer and to enable the silicon-metal contact.
BSF/back metal contact 116 is generally formed from aluminum (in the case of a p-type substrate) and is configured to create a potential barrier that repels and thus minimizes the impact of minority carrier rear surface recombination. In addition, Ag pads [not shown] are generally applied onto BSF/back metal contract 116 in order to facilitate soldering for interconnection into modules.
However, the use of aluminum may also be problematic for multiple reasons. As a result of thermal expansion mismatch between the silicon wafer and the aluminum layer, an aluminum BSF tends to cause solar cell warping, which leads to difficulties in subsequent production processes and decreases the yield due to increased breakage. Aluminum is also a poor reflector for the red light that is not absorbed by the wafer, reducing the solar cell efficiency. In addition, aluminum generally provides sub-optimal passivation to the substrate rear surface.
One solution may be to replace the blanket aluminum with a more reflective and better passivated layer in order to reduce charge carrier recombination and increase the absorption of long wavelength light. Additionally, the rear metal contact area may also be reduced to further optimize charge carrier recombination.
Solar cells configured with this architecture are commonly referred to as PERC (Passivated Emitter and Rear Cell) an architecture that was first introduced in 1989 by the University of New South Wales [A. W. Blakers, et al., Applied Physics Letters, 55 (1989) 1363-1365]. The devices fabricated in that study used heavily doped substrates as well as numerous expensive processing steps that are not compatible with high throughput manufacturing. Other versions of this cell architecture were later introduced as options to further increase the efficiency. Most notable among them is the PERL (passivated emitter rear locally diffused) [A. Wang, et al. J. Appl. Phys. Lett. 57, 602, (1990)], PERT (passivated emitter, rear totally diffused) [J. Zhao, A. Wang, P. P. Altermatt, M. A. Green, J. P. Rakotoniaina and O. Breitenstein, 29th IEEE Photovoltaic Specialist Conference, New Orleans, p. 218, (2002)], and PERF (passivated emitter rear floating junction) cells [P. P. Altermatt, et al. J. Appl. Phys. 80 (6), September 1996, pp. 3574-3586]. Similar to the original PERC cell, these architectures are expensive to manufacture. Since their introduction there have been numerous attempts to develop an industrially viable approach to make these cells.
In an alternate configuration, a selective emitter solar cell architecture on the front of the wafer may be used to further optimize solar cell efficiency. A selective emitter uses a first lightly doped region optimized for low recombination, and a second heavily doped region (of the same dopant type) optimized for low resistance ohmic metal contact.
Referring now to FIG. 2, a simplified diagram is shown of a solar cell with rear passivated and reduced rear area metal contact on a p− (boron doped) substrate 210 with an n+ (phosphorous doped) emitter region 220.
Here, a set of front metal contacts 222 connects to n+ emitter region 220 through front surface SiNx layer 219 in order to form an Ohmic contact. SiNx layer 219 is generally configured to passivate the front surface as well as to minimize light reflection from the top surface of the solar cell.
Likewise, the set of back metal contacts 216 connects with substrate 210 through back surface passivation layer 214 (such as SiNx) in order to also make an Ohmic contact.
However, the solar cell conversion efficiency of this architecture may also be problematic. For example, the presence of a metal layer in direct contact with the weakly-doped base wafer will tend to result in high contact resistance (i.e., a non-Ohmic contact). In addition, direct contact between n+ layer 212 (a byproduct of the POCl3 diffusion process) and the set of back metal contacts 216 will also tend to result in a shunted junction that further reduces device efficiency.
One solution may be to use a doping paste to form a localized p+ (heavily doped) region between n+ layer 212 and the set of back metal contacts 216 in order to minimize detrimental shunting. However, the use of conventional dopant pastes is problematic since they are generally comprised of SiO2 matrix with an addition of dopant containing compounds (see U.S. Pat. No. 4,104,091 and U.S. Pat. No. 6,695,903).
Aside from detrimental auto doping (the creation of volatile dopant species which dope unwanted surface areas away from the intended deposition area), conventional doping pastes are generally unable to mask ambient POCl3 (the absence of which would counter-dope the local region to a detrimental n-type and thus shunt).
In addition, because glasses (such as SiO2) tend to reflow at the temperatures required for dopant diffusion, and because this temperature is further reduced by the addition of dopants, it is difficult to created patterned features with traditional dopant pastes. Furthermore, because the paste matrix is silicon oxide, conventional dopant pastes are generally not compatible with HF-based acidic chemistries typically used to clean the substrate surface after paste deposition and prior to the diffusion process.
In view of the foregoing, there is a desire for a doping paste that is resilient to high temperature oxidizing processes (such as the POCl3 diffusion process), is able to mask ambient POCl3, and is compatible with HF-based acidic cleaning chemistries.